The pancreatic beta cell plays a pivotal role in the pathogenesis of Type 2 diabetes mellitus (T2DM). In states of increasing peripheral insulin resistance, there is a heightened need to synthesize and secrete insulin in order to maintain euglycemia. As a result of increased metabolic demand, the beta cell undergoes both a functional (insulin secretory) and proliferative (growth and replication) adaptive response. Unfortunately, the compensatory ability of the beta cell is finite and ultimately fails at some critical juncture, resulting in T2DM. Data suggest the ability (or inability) of the pancreatic beta cell to maintain this adaptive program is a key determinant of whether an insulin resistant individual will progress to frank hyperglycemia and diabetes, yet currently there are no effective clinical treatments that specifically target beta cel health in T2DM. The long-term goal of this applicant is to define the molecular pathways that contribute to altered beta cell compensation and function in T2DM. In order to produce and release finely regulated amounts of insulin, the mammalian beta cell possesses a highly developed endoplasmic reticulum (ER). The sarco-endoplasmic reticulum calcium ATPase (SERCA) pump resides in the ER membrane and is tasked with maintaining a steep calcium concentration gradient between the cytosol and ER lumen. This gradient is important for multiple Ca2+-regulated signaling pathways within the beta cell. Our published data show that expression and activity of the predominant beta cell isoform, SERCA2b, is markedly dysregulated in long-standing T2DM leading to profound changes in insulin secretion, ER health, and ultimately beta cell survival. In contrast, our preliminary data demonstrate that SERCA2b is upregulated in early diet-induced obesity, and differences in SERCA2b expression may underlie the ability of the beta cell to successfully mount a compensatory program. This work will test the hypothesis that SERCA2b and the maintenance of a robust ER calcium pool are uniquely required for both the initiation and propagation of the beta cell adaptive response to emerging insulin resistance. In Aim 1, the role of SERCA2b in beta cell compensation will be defined using in vivo and in vitro models of developing and advanced T2DM as well as a model of beta cell SERCA2b deletion. In Aim 2, the specific signaling and proliferative pathways that regulate SERCA2b and plasticity of the ER calcium pool as part of this adaptive program will be identified. Here, the relationship between calcineurin and IRS-2-mediated signaling pathways and SERCA2b expression and activity will be defined. The overall impact of this proposal will be the identification of key pathways that can be targeted clinically as a means of preserving beta cell function in T2DM.